special feature article
A newspaper or magazine article that covers a particular topic in depth, often as part of a planned series or special section.
This morning's newspaper carried a special feature article on environmental issues.
This magazine runs a special feature article on health every month.
特集 refers to the special feature or special edition itself (e.g., a TV special or magazine special), while 特集記事 specifically means the article within that feature.
記事 is a general article or news story, whereas 特集記事 is a special feature article that is part of a planned series or special coverage.
Compound of 特集 (とくしゅう, 'special feature') and 記事 (きじ, 'article').